1. Field of the Invention
The present invention relates to a segmented composite rotor; and more particularly, to a rotor for a permanent magnet, dynamoelectric machine, a method for fabricating such a rotor to provide it with improved strength and reliability for operating the machine at high rotational speeds, and a dynamoelectric machine incorporating such a rotor.
2. Description of the Prior Art
The electric motor and generator industry is continuously searching for ways to provide dynamoelectric, rotating machines with increased efficiencies and power and torque densities. As used herein, the term “motor” refers to all classes of motoring and generating machines which convert electrical energy to rotational motion and vice versa. Such machines include devices that may alternatively function as motors, generators, and regenerative motors. The term “regenerative motor” is used herein to refer to a device that may be operated as either an electric motor or a generator. A wide variety of motors are known, including permanent magnet, wound field, induction, variable reluctance, switched reluctance, and brush and brushless types. They may be energized directly from a source of direct or alternating current provided by the electric utility grid, batteries, or other alternative source. Alternatively, they may be supplied by current having the requisite waveform that is synthesized using electronic drive circuitry. Rotational energy derived from any mechanical source may drive a generator. The generator's output may be connected directly to a load or conditioned using power electronic circuitry. Optionally, a given machine is connected to a mechanical device that functions as either a source or sink of mechanical energy during different periods in its operation. The machine thus can act as a regenerative motor, e.g. by connection through power conditioning circuitry capable of four-quadrant operation. In a generator application, the mechanical device is often termed a “prime mover.”
Rotating machines ordinarily include a stationary component known as a stator and a rotating component known as a rotor. The rotor is ordinarily mounted on a shaft and supported for rotation in facing relationship with the stator. The shaft is connected to a load or a prime mover for transmission of rotational mechanical energy and the associated torque.
Usually, the rotor and stator are held in position by a frame in such a fashion as to permit continuous mechanical rotation of the rotor relative to the stator. The rotor is ordinarily associated with a shaft supported by bearings that support the rotor for its rotation and constrain the rotor against forces tending to cause the shaft to move either radially or axially. The rotor and shaft may be constructed as an integral assembly, or they may be separate parts secured by fasteners, press fitting, or other known means providing an attachment sufficiently robust to permit torque transfer between the rotor and shaft. It will be understood by those skilled in the art that a rotating machine may comprise plural, mechanically connected rotors and plural stators.
Virtually all rotating machines are conventionally classifiable as being either radial or axial airgap types. That is to say, adjacent faces of the rotor and stator are separated by a small airgap traversed by magnetic flux linking the rotor and stator. A radial airgap type is one in which the rotor and stator are separated radially and the traversing magnetic flux is directed predominantly perpendicular to the axis of rotation of the rotor. In an axial airgap device, the rotor and stator are axially separated and the flux traversal is predominantly parallel to the rotational axis. Axial airgap motors are often called disk or pancake motors, reflecting their short aspect ratio. That is to say, in these designs, the ratio of length along the shaft direction to the overall diameter is ordinarily much lower than in radial machines, which usually have the overall form of an elongated cylinder.
U.S. Pat. No. 6,995,489 to Ehrhart et al. provides a construction for an internal, permanent magnet rotor for a radial airgap electric machine. In an embodiment, a generally cylindrical rotor comprising a plurality of rotor permanent magnets is encircled about its circumference by a bandage, for which ceramic fiber is a preferred material. The fibrous material is said to provide strength in the circumferential direction and to maintain structural integrity of the rotor against centrifugal forces. A circumferential fibrous material is also disclosed for an external permanent magnet rotor for a radial airgap machine. Fibers externally wound around radial airgap rotors are also disclosed by Japanese Patent Publication JP10210690 to Kawamura, U.S. Pat. No. 6,751,842 to Roesel, Jr., et al., and U.S. Pat. No. 7,098,569 to Ong et al. U.S. Pat. No. 6,674,214 to Knörzer et al. provides an axial airgap machine in which the rotor comprises permanent magnet pieces embedded in a fiber- or fabric-reinforced plastic. U.S. Pat. No. 5,982,070 to Caamano provides a dielectric housing in which stator magnetic pieces are disposed.
Although the preponderance of present commercial devices have radial gap designs, there has been renewed interest in the potential of axial airgap machines for some applications, especially for situations in which the short aspect ratio geometry is more favorable. FIG. 1 depicts one form of permanent magnet rotor useful in an axial airgap, dynamoelectric machine. The power and torque capabilities of axial airgap devices can be increased either by increasing the device diameter or by employing a stack of multiple rotors and stators.
However, rotor design and construction are generally regarded as more difficult in the axial geometry. High speed designs are considered particularly challenging as a result of the combined axial and radial forces encountered during operation. Designers must carefully design rotors that are not prone to excitation of any normal modes that might lead to excess vibration, or even catastrophic mechanical failure, under foreseeable operating conditions.
Except for certain specialized types, motors and generators generally employ soft magnetic materials of one or more types. By “soft magnetic material” is meant one that is easily and efficiently magnetized and demagnetized. The energy that is inevitably dissipated in a magnetic material during each magnetization cycle is termed hysteresis loss or core loss. The magnitude of hysteresis loss is a function both of the excitation amplitude and frequency. A soft magnetic material further exhibits high permeability and low magnetic coercivity. Motors and generators also include a source of magnetomotive force, which can be provided either by one or more permanent magnets or by additional soft magnetic material encircled by current-carrying windings. By “permanent magnet material,” also called “hard magnetic material,” is meant a magnetic material that has a high magnetic coercivity and strongly retains its magnetization and resists being demagnetized. Depending on the type of motor, the permanent and soft magnetic materials may be disposed either on the rotor or stator.
A number of applications in current technology, including widely diverse areas such as high-speed machine tools, aerospace motors and actuators, and compressor drives, require electrical motors operable at high speeds (i.e., high rpm), many times in excess of 15,000-20,000 rpm, and in some cases up to 100,000 rpm. High speed electric machines are almost always manufactured with low pole counts, lest the magnetic materials in electric machines operating at higher frequencies experience excessive core losses that contribute to inefficient motor design. This is mainly due to the fact that the soft material used in the vast majority of present motors is a silicon-iron alloy (Si—Fe). It is well known that losses resulting from changing a magnetic field at frequencies greater than about 400 Hz in conventional Si—Fe-based materials causes the material to heat, oftentimes to a point where the device cannot be cooled by any acceptable means.
However, further problems arise in electric machines from circulating eddy currents. These problems are accentuated in devices operating at high frequencies and high speeds. In accordance with Faraday's law, eddy currents are induced in conductive elements of the rotor, shaft, and associated structure as a consequence of time-varying magnetic flux threading these components.
More specifically, as the rotor rotates relative to the stator, the rotor magnets experience cyclic differences in permeance coefficient during the course of each rotation, as the rotor magnets alternately pass between alignment with the teeth of the stator core and positions centered in the gaps between the stator teeth. As a result of this variation in permeance, flux within the rotor magnets and associated structure changes, thus inducing eddy currents.
Circulating eddy currents bring a number of undesirable effects. In some cases, they are high enough to cause significant heating in the rotor. The heating, in turn, is likely to cause irreversible loss of magnetization of the rotor permanent magnets, thereby drastically reducing device output. In extreme cases, the heating may even be severe enough to reduce the lifetime of the rotor magnets or destroy them.
A potentially even more serious problem arises from any current paths that traverse the shaft bearings. Without being bound by any theory, it is believed that such shaft currents arise from eddy current effects induced in accordance with Faraday's law. Inevitably, shaft currents cause sparking and gradual erosion of bearing surfaces. This in turn results in excess friction that can cause the bearings to overheat and even fail catastrophically.
To counter this danger, machine designs often incorporate a conductive path provided by brushes engaged with slip rings to provide an alternate path diverting any circulating shaft currents away from the bearings. This expedient may protect the bearing surfaces, but does not eliminate heating and its effects, not least the loss of energy efficiency. Moreover, the brushes and slip rings themselves are expensive, require regular maintenance or replacement, and occupy valuable shaft space. Rotor designs that minimize or eliminate deleterious shaft eddy current effects are thus highly sought.
Accordingly, there remains a need in the art for electrical devices that are simple and economical to construct and provide high operational efficiency. Ideally, an improved machine would provide higher efficiency of conversion between mechanical and electrical energy forms by using low-loss soft magnetic material and advanced, high energy product permanent magnet materials. An ideal machine would also provide high power and torque density computed on either a volumetric or mass basis, and with or without liquid or gas cooling. Improved efficiency in generating machines powered by fossil fuels would concomitantly reduce air pollution. The machine would be smaller and lighter and would satisfy more demanding requirements of torque, power, and speed. Cooling requirements would be reduced. Motors operating from battery power would operate longer for a given charge cycle. For certain applications, improved axial airgap machines are especially desired because of their size and shape and their particular mechanical attributes.